The Collaborative International Dictionary of English v.0.48:

Extenuation \Ex*ten`u*a"tion\, n. [L. extenuatio: cf. F.
   ext['e]nuation.]
   The act of axtenuating or the state of being extenuated; the
   act of making thin, slender, or lean, or of palliating;
   diminishing, or lessening; palliation, as of a crime;
   mitigation, as of punishment.
   [1913 Webster]

         To listen . . . to every extenuation of what is evil.
                                                  --I. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extenuation
    n 1: a partial excuse to mitigate censure; an attempt to
         represent an offense as less serious than it appears by
         showing mitigating circumstances [syn: extenuation,
         mitigation]
    2: to act in such a way as to cause an offense to seem less
       serious [syn: extenuation, mitigation, palliation]